Suspension screen raking system

ABSTRACT

A screen raking system has a screen and a rake which is driven by a single hydraulic cylinder to move the rake up and down a length of a screen and also move the screening away from the screen, compress the screenings and move them through a waste pipe to an area remote from the screen for disposal.

FIELD OF THE INVENTION

This invention relates to a suspension screen raking system, by which ismeant a system for the removal of solids and/or floatables (referred togenerally herein as solids) from a screen placed across a channel orpool having a liquid suspension of such solids. The suspension screenraking systems to which the invention relates include water screenraking systems such as those placed across a water channel, such as asewerage or waste water treatment channel, or a channel supplying waterto a turbine.

BACKGROUND OF THE INVENTION

Suspension or water screen raking systems usually have screens composedof horizontally spaced upright bars which are placed across channels oracross an entrance or exit to a pool or reservoir. The screens areusually placed at an angle to the vertical and are provided to act as afilter. It is well known that the screens become blocked due tosuspended matter in the water, or other liquid which is being filtered,collecting across the bars and eventually providing a substantialimpediment to liquid flow through the screen.

It is known to utilise a rake or brush having teeth or brush elementswhich interdigitate with the screen bars to lift solids collecting onthe bars and thus prevent the screen becoming clogged. The rake usuallylifts the solids to the top of the screen where they are tipped into aremoval channel or otherwise disposed of.

Such rakes are usually driven by a motor through a system of gears orchains and sprockets. This is not an ideal arrangement. It is complexand prone to jamming and failure. The use of an hydraulic or pneumaticram would be preferable.

Existing systems also have problems with transferring the capturedsolids (commonly referred to as “screenings”) to a position where theycan be further processed. Usually, such further processing includespressing the screenings to reduce bulk prior to transporting to alocation where they can be disposed of. Existing designs for washing andreducing screenings are maintenance intensive with many moving parts.Others are limited by their ability to cope with the large rise and fallin liquid levels within the channel, that is, peak loadings or flows.The inability of prior art constructions to cope with the larger risesand falls in liquid levels can create further problems associated withhead loss. It would therefore be desirable to have a system which iscapable of drawing screenings together to reduce their bulk for disposalin a manner, which is simple, effective and substantially maintenancefree.

There is a considerable requirement in the water supply and waste waterdisposal and treatment industries for the supply of small to mediumscreening systems such as those, which could al most, screen a flow ofup to 1 or perhaps 2 cubic meters per second, and which are convenientlylocated in the water channel.

OBJECT OF THE INVENTION

It is an object of the present invention to provide suspension screenraising systems or methods which will at least go some way towardovercoming disadvantages associated with the prior art, or which willalternatively at least provide the public with a useful choice.

An alternative object of the invention is to provide a racing systemwhich is on one plane or axis with a single prime mover.

Another alternative object of the invention is to provide a rakingsystem wherein a single prime mover rakes a screen and conveysscreenings to a disposal point remote from the screen.

SUMMARY OF THE INVENTION

Accordingly in one aspect the invention consists in a system for theremoval of solids from a suspension in which the solids are provided,the system including

-   -   a screen having a plurality of apertures therein,    -   a rake provided on or adjacent to the screen,    -   driving means to cyclically drive the rake in two opposing        directions relative to the screen to move solids captured on the        screen toward one end of the screen.

Preferably the screen comprises a trough.

Preferably the flow of solids from the screen through to discharge is insubstantially one direction or axis.

Preferably the driving means comprises a single prime mover.

Preferably the screen is concave.

In this documents reference to “concave” means that the screen has edgeswhich are nearer 10 the direction from which the water or othersuspension is flowing than a part of the screen between the edges.Therefore, the screen may be a “V” shape or a “U” shape incross-section. It may also have a substantially flat mid section withvertical or angled sides.

Preferably the driving means moves the rake periodically back and forthalong the screen.

Preferably the time taken for the rake to traverse the screen in adirection toward one end is longer than the time taken on the downwardstroke is less than the time taken for the rake on the upward stroke,

Preferably the accumulator means comprises a transition, preferably acone transition element through which the solids are compressed beforereaching an outlet like for disposal.

Preferably the screen is substantially semi circular. Preferably thescreen extends across approximately 60%-70% of the circumference orthough an arc of approximately 210° to 260°.

Preferably the screen sits at an operating angle which is suited to theapplication requirement, and may for example be from 20 to 45 degreesfrom the horizontal.

Alternatively, the screen is substantially triangular in cross-section.

Alternatively, the screen is substantially square in cross-section.

Preferably the screen elements run longitudinally with slots which arekept clear by a rake positioned and installed having teeth radiatingfrom a disk member. Preferably the slots are greater than or equal to 3mm.

Preferably the screening elements have a profile comprising a verticalsection with a small section al right angles thereto.

Preferably the screening elements have a profile corresponding to aninverted “L”. Alternatively, the screening elements in profile comprisea T-section or alternatively a Y-section or have a vertical sectionwhich tapers from a wide cross-section to a narrow cross-section towardsthe base thereof.

Alternatively, the screen comprises a perforated member.

Preferably the perforated member has tapered apertures.

Preferably rake guide slots are also provided.

Alternatively, the drive means comprises a worm drive or other speedreducer, for example a speed reducer operating through a rack and pinionarrangement.

Preferably a bin or collector is provided to collect the screeningsdischarged from the accumulator means.

Preferably a wedge means is provided for moving screenings to adischarge location.

Preferably a plough means is provided for moving screenings through theaccumulator means.

Preferably the bin may be up to 6 metres from the screen and 2-3 metresabove the screen.

In a further aspect the invention consists in apparatus for the removalof solids from a screen, the solids having been separated from asuspension by the screen, the apparatus including

-   -   a rake means to, in use, lie adjacent to or in contact with the        screen,    -   a driving means for moving the take means relative to the screen        to move solids collecting on the screen, in use, to one end of        the screen, and    -   an accumulator means provided at one end of the screen, the        accumulator means reducing in cross-sectional area with distance        from the screen whereby solids delivered to the one end of the        screen by the rake means are pressed into close proximity with        each other.

In a further aspect the invention consists in a system for removal ofsolids from a liquid suspension in which the solids are provided, asystem including

-   -   a screen comprising horizontally spaced bars,    -   a rake provided on or adjacent to the screen,    -   a driving means to move the rake relative to the screen to        thereby rake solids captured on the screen toward one end of the        screen the driving means comprising a hydraulic or pneumatic        cylinder which directly drives the rake.

In a further aspect the invention consists in apparatus for removal ofsolids from a screen, the solids having been separated from a suspensionby the screen, the apparatus including

-   -   a rake means to, in use, lie adjacent to or in contact with the        screen    -   a driving means for moving the rake means relative to the screen        to move solids collecting on the screen to one end of the        screen, and    -   the rake means including retraction means whereby the rake means        may be moved between an operative position for moving solids and        an inoperative position whereby the rake means may be moved        relative to the screen by retraction means without substantially        moving solids on the screen.

DRAWINGS AND DESCRIPTION

The invention will be described with reference to drawings appendedhereto in which

FIG. 1 is a side elevation of screening apparatus according to thepresent invention,

FIG. 1 a is a side elevation of a screw conveyor adapted to be fittedinto the transition funnel and waste pipe of FIG. 1,

FIG. 2 is a partial plan view of the apparatus of FIG. 1,

FIG. 3 is an end elevation of the apparatus of FIG. 2 viewed from thedirection of arrow A,

FIG. 4 is an end view of the apparatus of FIG. 2 when viewed from thedirection of arrow B,

FIGS. 5A-5D show screen elements according to the invention incross-section,

FIG. 6 is a side elevation of further screening apparatus according tothe invention including a rake profile and baffle to assist clearance ofthe screen during the downward stroke,

FIG. 7 is a side elevation of further screening apparatus according tothe invention including a retraction device for selectively retractingthe rake teeth,

FIG. 8 is a plan view of the lower part of the screen showing the teethin the extended position,

FIG. 9 is a plan view of the upper end of the screen showing the raketeeth in the retracted position,

FIG. 10 is an end elevation of the rake arrangement used in FIGS. 7 to9,

FIG. 11 is a side elevation in cross-section of a helical flight similarto that described with reference to FIG. 1A, but in a trough dischargetube,

FIG. 12 is an end elevation of the apparatus shown in FIG. 11,

FIG. 13 is a side elevation of the apparatus of FIGS. 11 and 12 butfurther showing a drive arrangement for driving the helical flight andalso showing the prime mover which drives the rake,

FIG. 14 is a side elevation in partial cross-section of another exampleof a rake system according to the invention,

FIG. 13 is a side elevation of a wedge member used in the-constructionof FIG. 14,

FIG. 16 is an end elevation of FIG. 15,

FIG. 17 is a side elevation of a plough member used in the constructionof FIG. 14,

FIG. 18 is a side elevation in cross-section of a bearing used in theconstruction shown in FIG. 14,

FIG. 19 is a front elevation of FIG. 18,

FIG. 20 is an end elevation of the system of the present invention butwith a perforated plastic screen,

FIG. 21 is a plan view of the screen of FIG. 20 shown holes therein,

FIG. 22 is a partial cross-section through A-A of FIG. 21, and

FIG. 23 is an expanded view of FIG. 20 showing holes in the screen ingreater detail.

DESCRIPTION OF PREFERRED EMBODIMENT

The following description is given by way of example and illustrationonly and shall not be used to limit the scope of the invention.

Reference will be made to the drawings, wherein like numerals designatecorresponding parts throughout the several figures.

EXAMPLE 1

Referring to FIG. 1, the apparatus according to the present invention asshown generally referenced A. The apparatus has one or more (preferablytwo) guide rails 1 which in use support a rake 3 which is mounted on adisk 2. The disk 2, and rake tines/teeth 3 are illustrated more clearlywith reference to FIG. 3. However, continuing to refer to FIG. 1, therail 1 has an end guide brackets 4 thereon, which define the upper andlower positional limits or movement of the disk guide 5 which in use,slides upon guide rail 1 and supports the rake support disk 2. In FIG.1, the rake 3 is shown in two positions, an upper position referenced 8,and a lower position near the lower end of the guide rail. The twoextreme positions of the rake movement are shown for illustrativepurposes only, there being only one rake disposed on the guide rails 1in use. A centre guide 6 is also provided for the rake and prevents anysubstantial transverse movement of the rake tines or teeth, but allowslongitudinal movement of the rake 3 in a motion parallel to thedisposition of the guide rails 1.

The screen elements 7 are longitudinal elements which, when assembledparallel to each other, as shown in the drawings, effectively create thescreen which is generally referenced 25. FIG. 1 shows only one screenelement 7 for clarity. For the purposes of the present example, in whicha device for relatively low flow rates (in the order of 1 or 2 cubicmetres per second) is described, the gap between adjacent longitudinalscreen 25 elements is preferably 3 mm or more. The longitudinal elementsmay be a variety of shapes, but preferred shapes are illustrated anddescribed further below with reference to FIGS. 5A to 5D. The screen,and preferably, but not necessarily, the apparatus that drives the rakeare provided at an operating angle suited to, the applicationrequirement, which will usually be between 25 to 45 degrees to thechannel invert i.e. usually 25 to 45 degrees to the horizontal. Thescreen elements are preferably made from steel, but those skilled in theart will appreciate that suitable plastic materials such aspolypropylene could alternatively be used. In particular, the screen maycomprise a perforated plastic structure as described below withreference to FIGS. 20-22. Also, although reference is made throughoutthis document to the screen being constructed from individual elongateelements, the examples provided may also use a screen that isconstructed as a perforated object. For example, a sheet material suchas a sheet of steel may be perforated and formed to construct thescreen. Also, a screen could be moulded from a plastics material, themould providing the required perforations in the resultant screenobject. The perforations may be in the form of elongate slots but couldalso conform to other shapes.

A hanger bracket 9 is provided from which a transition funnel 10 isprovided. The funnel 10 is generally in the form of a frustum of a cone,but it will be seen that other shapes could be used according to thepresent invention. The transition funnel 10 provides an effective way ofaccumulating screenings and reducing then by squeezing out liquid forexample. The larger end of the transition funnel 10 is open to the upperend of the travel of the rake assembly. The smaller diameter end of thetransition funnel 10 is connected to the discharge pipe 11. The rakeassembly is driven by a hydraulic shaft 12, which has a piston orotherwise co-operates with a double acting hydraulic cylinder 13whereby, fluid is provided to and/or from hydraulic or pneumaticcircuits to cylinder 13 to create movement of the shaft 12 relative tothe cylinder and thereby move the rake in a periodic reciprocatingmotion up and down the length of guide rail 1. The rake and screen 25assembly are supported by a support arrangement Generally referenced 14.

The power cylinder 13 may be powered by any suitable available medium,for example hydraulic, pneumatic, or steam. The cylinder is arranged tomove the shaft to translate the rake cyclically to the top and then backto the base of the screen. Shoes or linings, such as plastic linings inguide 5 (not shown for clarity) assist in reducing friction between thestationary and moveable assemblies.

The cylinder 13 is double acting, or there are two cylinders, or thereis an appropriate mechanical arrangement associated with a singlecylinder to achieve the desired result of a reciprocating cyclical rakeaction. Furthermore, the required control apparatus for activating thecylinder is provided, as is well known to those skilled in the art

As a further alternative, the motive power to drive the shaft 12 couldbe provided by a motor with a speed reducer operating through a rack andpinion arrangement, or a worm drive arrangement.

The shaft 12 is preferably provided so as to directly drive the rake andto pass through the waste pipe 11 and transition funnel 10. However, itwill be seen that other arrangements may be provided without departingfrom the scope of the invention. In particular, the shaft 12 may beprovided above the pipe 11 and transition funnel 10 and have a furtherarm or a bend that connects the shaft 12 to the rake 3. Also, the rake 3may be driven indirectly by another mechanical arrangement if desired.

Referring now to FIG. 1 a, optionally, a screw conveyor 101 can beincorporated into the existing design for picking up the screenings fromthe upper end of the rake assembly within the transition funnel 10 andtransporting them to the end of the waste pipe 11 or any other selectedlocation where the screenings are discarded. Typically these screeningswould most likely originate from a sewerage discharge or from anindustrial discharge. The helical flight of screw 105 is mounted arounda worm conveyor shaft 104, which is a hollow shaft driven be a shaftmounted speed reducer 106. The speed reducer 106 is provided to drivethe helical screw at different speeds depending on the nature of thescreenings or the work rate (i.e. cycle speed) of The rake. The shaft12, which powers the rake assembly passes right through the entirelength of the worm conveyor shaft 104. The main function of the screwconveyor 101, which operates in concert with the rake system, is to movethe screenings through the transition funnel (where some compression ofthe screenings occurs) and through the discharge pipe 11 for disposal.

Alternatively, the transition funnel 10 and waste pipe 11 can beexcluded from the design. In the absence or the screw conveyor, thescreenings build up in the funnel 10 under the action of the rake on therake upstroke and the rake action moves the screenings through thedischarge pipe 11 and into the bin 11 a. As a result, the screenings aredischarged from the upper end of the discharge pipe 11 directlyunderneath which a bin 11 a is provided to collect the discharge.

Turning now to FIG. 2, some further information is provided in thispartial plan view of the apparatus. It can be seen that two guide rails1 are clearly provided and the transition funnel is mounted at twohanger bracket supports 9.

Turning to FIGS. 3 and 4, it will be seen that the screen 25 is partcircular in this example and the rake assembly is correspondingly partcircular having tines or teeth 3 a which radiate from the periphery ofthe rake disk and interdigitate with the longitudinal screen elements 7of the screen 25. In FIGS. 3 and 4 the element support 15 and brace 16for holding the rake disk relative to the guide members 1 is shown.

The longitudinal screen elements 7 may be a number of different shapesor cross-sections and some of these are illustrated in FIGS. 5A to 5D,the preferred shape being that shown in FIG. 5B. As can be seen from thepartial cross-section of FIG. 3, the longitudinal screen elements 7having the cross-section according to FIG. 5B are organised with theshort leg of the “L” being approximately 80° relative to the centrepiece, and facing upwardly or inwardly toward the rake disk. In FIG. 5Athe profile has a vertical section with a smaller section at rightangles intersected at the centre. This alternative is referred to as a“piece”.

In FIG. 5C the element has an end piece with sloping shoulders. Thisalternative is referred to as a “Y wire”.

In FIG. 5D the element has a gradual taper from top edge to the loweredge.

The operation of the apparatus will now be described. The apparatus isarranged so that a flow of waste liquid or water which includes asuspension of solid matter (which may include floating solid matter) isdelivered, as illustrated by arrow 17 in FIG. 1, so that the liquid flowis through the gaps between longitudinal screen elements 7. The inlet 26flow area to the screen 25 is preferably approximately 33% of the totalarea of screen 25. It will be seen that the screen 25 is advantageouslyarranged so as to assist capture of solids in the liquid flow. That isto say, the screen 25 is in the form of a trough, being substantially“concave” having a centre portion which when viewed in cross-section issomewhat lower than the side portions of the screen 25. This generalshape may be curved as illustrated in the preferred embodiment of FIG.1, but will be seen that other shapes may accommodate the same purpose,for example a screen 25, shape which is substantially rectangular incross-section either having vertical or angled or curved sides, or ascreen 25 shape which simply has angled sides or is triangular or “V”shaped in cross-section. All of these shapes which have sides of somesort to the screen 25 substantially prevent any solids from escapingfrom the sides of the screen 25 and also generally assist the rakingprocess by tending to have solids collect in the centre of the screen 25under gravity as the rake moves. The screen shapes described above alsohave the advantage that a larger screen area is provided in a reducedlateral space.

As illustrated in FIG. 1, as the rake moves upwardly in the direction ofarrow 18 following actuation of the cylinder 13, the rake tines or teeth3 a scrape, rake or otherwise capture or move solid matter up the screen25 towards the upward delivery end of the screen 25 where the solids aredeposited in the transition funnel 10. Upon reaching the upper limit ofthe screen 25, the rake returns down the screen 25 to the lowerposition. In the preferred embodiment, the rake travels up the screen 25more slowly than it travels in the reverse direction toward the bottom.Preferably, the rake travels down the screen 25 at approximately ⅓ thetime it takes to travel up the screen 25. The cycle time of the rake canbe varied by the control system dependent on the flow rate through thescreen and/or the solids content of the through flow. By way of examplea typical time period for the upstroke may be 20 seconds, and thedownstroke 7 seconds.

Any solid matter which is captured on the screen 25 while the rake is atthe upper end of the screen 25 is brushed off the screen 25 by the rakewith its relatively quick downward movement. Usually, matter which ismoved by the rake with the downward movement will simply resurface fromthe liquid flow to be again deposited in the centre of the screen 25 andtherefore be collected by the rake on its upward stroke. However, if thesolid matter in the liquid flow is such that it tends not to bere-supplied to the screen 25 upon the downward stroke or the rake, othermeans may be used to dislodge the solid matter from the base of thescreen 25 so that it may be returned to the liquid flow to be depositedagain above the rake. For example, an appropriate mechanism forreturning such solids in this manner may be bubbling air or other gas ordirecting an appropriate flow of liquid around the base of the screen 25elements to force a general upward flow of any solids back into thewater streams. It will be seen that other devices or methods could beused. For example, a plate or baffle (not shown) may be providedattached to the bottom of the rake and extending generally upwardly fromthe rake so as to assist dislodgement of solids from the screen on thedownstroke, and assist collection of solids on the upstroke. The plateor baffle may be hingedly attached to the rake (as described furtherbelow) so that it folds flat on the downstroke and returns to asubstantially upright position on the upstroke.

Once the solids have been pushed up to the transition funnel 10, theywill over time accumulate and become compressed by the transition funnelafter being pushed by farther solid matter which is dragged into thetransition tunnel by the rake. Therefore, over time, the general form ofthe transition funnel, being a shape which reduces in cross-sectionalarea, will tend to compress the collected screenings and force them intoa relatively compressed or compacted state into the discharge pipe 11.The screenings will continue to move through the discharge pipe 11around the arm 12, before being deposited from the end of the dischargepipe 11 into the waste bin. The general movement of screenings throughthe discharge pipe 11 is illustrated by arrow 19.

EXAMPLE 2

Turning now to FIG. 6, a further embodiment of the invention isillustrated. The overall apparatus is substantially the same as thatdescribed with reference to the preceding figures. Also, the referencenumerals in the embodiment shown in FIG. 6 which are the same asreference numerals used in the preceding figures designate the same orsubstantially similar features of both embodiments.

The apparatus of FIG. 6 mainly differs form the embodiment describedabove insofar as an improved arrangement for sweeping screenings off thescreen 25 on the downward smoke is provided. This further apparatusgenerally comprises a baffle member 22 and a specially angled lower edge23 on the reverse side of the rake. The angled lower edge 23 assistswith dislodging solid matter trapped on the screen during the downstrokeof the rake, so that the solid matter may be forced back in the liquidflow and thus captured on the next upstroke as shown by arrows 20 and17. A further aspect which may be provided in the embodiment shown inFIG. 6 is an improvement to the collection, accumulation and compressionaspect of the invention through the transition funnel 10 and thedischarge pipe 11.

The baffle member 22 is provided hingedly dependent from the bottom ofthe rake, preferably, adjacent to the disc guides 5 of the rake. Thebaffle 22 is preferably shaped so as to be substantially the same shapeas the rake (in the present example being substantially disk shaped).The baffle member 22 is also allowed to simply “float” but is preferablycounter balanced, so as to remain in a position as illustrated in FIG. 6when the rake is traversing its downward stroke. When the rake movesthrough its upward stroke, the baffle member 22 moves into a positionwhere it lies flat against the rake tines 50 as to assist transport ofsolid matter collected off the screen 25 by the Take towards the top endof the screen 2. In this way, the baffle member 22 assists intransporting solid matter up the screen 25 and it also assists indelivering solid matter into the transition funnel 10. It will be seenthat the lower front edge of the rake is also angled and this furtherassists in collecting screenings from the screen 25 surface,particularly any solid matter, which becomes wedged between thelongitudinal screen 25 elements,

On the downward stroke, with the baffle member 22 in position shown inFIG. 6, the angled edge 23 of the lower edge of the rake lifts the solidmatter collected on the screen 25 while the rake has been at the upperend of the screen 25. The solids lifted by the reverse side of the rakewill mainly be returned into a part of the screen 25 above the rake bymeans of the flow of liquid across the screen 25. This is illustrated byarrow 21 in FIG. 6. Alternatively, solids which are too large in size tomove through the rake and back onto the screen 25, will be transportedby the rake down to the base of the screen 25 so that they are returnedto the flow of liquid into the screen 25 above the rake. This path oftravel is indicated by arrow 20 in FIG. 6. Therefore, it will be seenthat an effective and simple way of ensuring that solids whichaccumulate on the screen 25 while the rake is near the top of the screen25 are returned to the main flow path.

Referring now to the shaft 12, it will be seen that a member 30 isprovided about shaft 12 in the vicinity of transition funnel 10. Thismember 30 is generally cone shaped, but it will be seen that othershapes may be provided. The purpose of this member 30 is to ensure thatcontinued travel of the screenings compressed by the transition funnel10 is facilitated through to the outlet tube 11. Therefore, thesubstantially perpendicular top face of the cone 30 provides arelatively flat surface against which screenings are moved upwardly. Thefront surface is angled so as not to encourage flow of screenings backtowards the screen 25. Also, a compression member comprising a furtherbaffle 32 which is preferably sprung or otherwise tensioned or biasedtoward the end of outward pipe 11 can assist to ensure that thescreenings are further compressed before they are released into the bin11 a.

EXAMPLE 3

The following example should be regarded as the most preferredembodiment of the invention.

Turning now to FIG. 7, a further side elevation of the apparatusgenerally described in the preceding figures is shown with the additionof a mechanism for extending and retracting the rake teeth. Thereference numerals used on the embodiment shown in FIG. 7 that are thesame as reference numerals in the embodiments described above depict thesame or substantially similar features.

The main difference with the rake assembly of the embodiment shown inFIG. 7 is that the rake has a guide block 38 which receives one end ofeach rake tooth 40. The other end 36 of each the rake tooth is theoperative part of the rake i.e. the part which actually moves solidsalong the screen. As will be seen, between ends 36 and 40 of each raketooth, the tooth is pivoted at a point 42. Therefore, when the driveshaft 18 is moved relative to the remainder of the rake assembly (i.e,when the rake changes direction for example) the driving block 38 willmove a short distance relative to the rest of the rake assembly becauseof the friction of the rake assembly on the screen.

As shown in FIG. 7, when the rake has reached the top of the screen andthe drive shaft 18 move in a direction back down the screen, drivingblock 38 will be moved relative to the remainder of the rake and willlift the teeth 36 so that they are in a position substantially parallelto the screen as shown at the upper end of the screen in FIG. 7.

Conversely, at the lower end of the screen, as shaft 18 begins to movein an upward direction to drag the rake up along the screen, the drivingblock 38 will move upwardly relative to the remainder of the rakeassembly which will pivot the teeth 36 downwardly so that theyinterdigitate with the screen elements ready to drag solids along thescreen to the upper end of the screen.

With this arrangement, it will be seen that the rake can be effectivelyretracted on the downward stroke so that solids are not dragged backdown the screen, but are only moved up the screen in one directiontoward the outlet for delivery to the bin.

Turning now to FIG. 8, a plan view of the lower end of the screen isshown, again with the rake teeth 36 in a position which they areextended ready to drag solids up the screen.

Turning to FIG. 9, a plan view of the upper end of the screen is shown,corresponding to the position of the rake at the upper end of the screenin FIG. 7. The teeth are shown in the retracted position.

Referring now to FIG. 10, the general arrangement of rake teeth in thesemi circular screen arrangement is illustrated.

Referring now to FIG. 11, a screw or worm 105 having a helical flight ismounted around a worm conveyor shaft 104 as described with reference toFIG. 1A. The only difference with this construction is that the wastepipe 11 instead comprises a trough 150. In FIG. 13, the screw drive 106and hydraulic cylinder 13 are shown.

Turning now to FIG. 14 a further example of the present invention isillustrated. In this example, a plough 160 is also provided dependentfrom the shaft 12. The plough 160 is shown in FIG. 14 in a lowerposition and in its uppermost, extended, position. As can be seen fromthe extended position plough placement in which it is within thetransition funnel 10, the plough facilitates movement of screeningswithin the transition funnel 10 towards the waste pipe 11 every time themain shaft 12 nears the upper limit of its extension. Therefore, everytime the rake moves near the top of the screen, the plough provides apushing action to push screenings through the transition funnel 10 andinto the discharge pipe 11. FIG. 14 also shows that a support bearing162 is provided for shaft 12. This bearing provides support for theshaft 12 which may be required in response to loads imposed on theplough 160 by accumulated screenings in the transition funnel 10 and/orwaste pipe 11.

Also, to further encourage movement of screenings through waste pipe 11,a wedge 164 is provided on shaft 12. The wedge is generally conical inform, or may be described as a frustom of a cone. The wedges outersurfaces are angled so that at minimal driving action is imposed onscreenings in the waste pipe 11 when the wedge is travelling in adownward direction. However, in the other direction, the wedge has anabrupt surface which is perpendicular to the movement of the wedge. Thisfacilitates movement of screenings through discharge pipe 11. Therefore,when the rake is travelling down toward the base of the screen,screenings which are in the waste pipe 11 are unlikely to besignificantly moved down the discharge pipe. However, when the rake istraveling towards the top of the screen, the wedge tends to movescreenings within the discharge pipe, up, along the pipe.

In FIG. 15, the wedge is shown in greater detail, and in FIG. 16 thewedge is shown in end elevation from which it can be seen that the wedgeis substantially conical in form

In FIG. 17 the plough is shown in side elevation including a mountingaperture 170 to mount the plough relative to the shaft 12.

In FIG. 18, the bearing is shown having an outer case 180 which issubstantially rigid, a cover 182 and a central bearing element 184.Cover members 182 and 180 securely hold bearing 184 in place. However,bearing 184 is made from a flexible plastics material, most preferablyultra high molecular weight polyethylene (UHMWPE). The inner surfaces ofbearing 184 contact shalt 12, but since the bearing is constructed froma resilient material, the bearing allows movement of the shaft relativeto the cover members 180 and 182. In this way, the shaft is supported,but there is still sufficient tolerance and flexibility for the ploughmember and unit as a whole to function effectively.

The bearing apparatus of FIG. 18 is shown in end elevation in FIG. 19.

As mentioned previously in this document, the screen may alternativelycomprise a perforated plastics structure. Such a screen is illustratedin FIGS. 20-22, and may be referred to as a plastic media perforatedscreen. The screen may be moulded in the required form. Alternatively,the screen may be constructed from a sheet of plastics material which istreated to give it the required overall shape, and machined to providethe required slots and apertures as described further below.

Referring to FIG. 20, the perforated screen 200 is shown in endelevation with partial view of the rake including rake tines or teeth 3a also illustrated. The holes provided in the screen 200 are not shownin FIG. 20, but are shown in FIGS. 21 and 22. In FIG. 20, thelongitudinal slots 210 that run the length of the screen are shown.These slots provide a guide for the rake teeth 3 a to run along thelength of the screen. Therefore, the rake can operate along the screenas described in the embodiments discussed above to push solids clear ofthe screen. The slots are each preferably approximately 10 mm wide inthe most preferred embodiment.

Turning, now to FIG. 21, the screen 200 is shown in plan view and theperforations in the screen are also shown. The perforations take theform of holes 212. These are preferably placed at 10 mm centres over thescreen. Furthermore, the holes are preferably tapered. This is shownmore clearly in the partial cross-section of FIG. 22. In that view, itcan be seen that The holes taper from a larger aperture on the rear sideof the screen to a smaller aperture on the front (i.e. the rake side) ofthe screen. In this way, material which has a dimension that is similarto that of the holes 212 sits on top of the holes rather than passinginto the holes and becoming jammed. If any material does just manage tofit through the entrance to one of the holes 212, then the path for thatpiece of material through the hole becomes progressively easier due tothe increasing taper towards the rear of the screen. Therefore,materials are unlikely to get stuck in the holes. If any material is notpassed through the holes, then it will be removed by the rake.

The holes 212 are shown in greater detail in FIG. 23. In particular, thetaper on the holes can he seen by the difference in size between theentrance aperture 214 and the exit aperture (-line) 216 for each hole.

It will be appreciated that the features referred to in the examplesabove may be interchanged amongst the various constructions to provide araking system which best meets the requirements of any particularinstallation.

It will be seen that the apparatus has the distinct advantage that thescreenings are allowed to compress and compact and are deliveredautomatically to a bin without need of any time consuming compaction orcompression operations and without requiring additional mechanicalapparatus to male the screenings away from the rake and deposit them ina waste receptacle such as a bin.

In addition the working components of the apparatus are accessible incomparison to the prior art, making it easier to maintain.

It will be appreciated that the invention maybe provided in differentembodiments without departing from the spirit or scope of the inventionas set forth herein. Where the word “comprises” or variance thereof suchas “comprising” are used throughout this document, it will be understoodthat this word is not being used in a limited sense, but is insteadbeing used in an inclusive sense.

1. Apparatus for the removal of solids from a suspension in which thesolids are provided, the apparatus including a screen having a pluralityof apertures therein, a rake provided on or adjacent to the screen,driving means to cyclically drive the rake in two opposing directionsrelative to the screen to move solids captured on the screen toward oneend of the screen.
 2. Apparatus as claimed in claim 1 wherein the flowof solids from the screen through to discharge is in substantially onedirection or axis.
 3. Apparatus as claimed in claim 1 wherein the screencomprises a trough.
 4. Apparatus as claimed in claim 1 wherein thedriving means comprises a single prime mover.
 5. Apparatus as claimed inclaim 3 wherein the screen is substantially concave.
 6. Apparatus asclaimed in claim 1 wherein the driving means moves the rake periodicallyback and forth along the screen.
 7. Apparatus as claimed in claim 6wherein the time taken for the rake to traverse the screen in adirection toward one end is longer than the time taken on the downwardstroke is less than the time taken for the rake on the upward stroke. 8.Apparatus as claimed in claim 1 including accumulator means toaccumulate the solids, the accumulator means comprising a transitionelement through which the solids are compressed before reaching anoutlet for disposal.
 9. Apparatus as claimed in claim 1 wherein thescreen is substantially semi circular and extends across an arc ofapproximately 210° to 260°.
 10. Apparatus as claimed in claim 1 whereinthe screen elements run longitudinally with slots which are kept clearby a rake positioned and installed having teeth radiating from a diskmember. Preferably the slots are greater than or equal to 3 mm. 11.Apparatus as claimed in claim 1 wherein the rake has tines which arecapable of being disposed in an operative position and in an inoperativeposition.
 12. Apparatus as claimed in claim 11 wherein the tines aredisplaced in the operative position when the rake travels in at upwarddirection, and disposed in the inoperative position when the raketravels in the downward direction.
 13. Apparatus as claimed in claim 1wherein a waste conduit is provided, and the driving means includes ashaft provided through the waste conduit.
 14. Apparatus as claimed inclaim 13 wherein the shaft includes a wedge to facilitate movement ofscreenings through the waste conduit.
 15. Apparatus as claimed in claim1 wherein the rake includes a baffle means to facilitate movement ofsolids toward the one end of the screen.
 16. Apparatus as claimed inclaim 1 wherein the rake includes a plough means to facilitate movementof solids off the screen.
 17. Apparatus for the removal of solids fromthe suspension in which the solids are provided substantially as hereindescribed.
 18. Apparatus for the removal of solids from the suspensionin which the solids are provided substantially as herein described withreference to accompanying drawings.
 19. Any novel feature or combinationof features disclosed herein.